1. Field of the Invention
The present invention relates to a charging device, which charges an image bearing member such as a photosensitive member and a dielectric member, and to an image forming apparatus such as a laser beam printer and a copying machine, to which the charging device is applied.
2. Description of Related Art
Conventionally, in an image forming apparatus such as a copying machine and a laser beam printer using an electrophotographic process, an image is formed by a series of image forming processes in general. The image forming process comprises a charging process, a latent image forming process, a developing process, a transfer process, a fixing process and a cleaning process. More specifically, in the charging process, the image bearing member such as a photosensitive drum and a dielectric member is charged evenly and uniformly, and in the latent image forming process, an electrostatic latent image is written into the image bearing member after being charged. Further, in the developing process, a toner is attracted to the electrostatic latent image so that the latent image is developed as a toner image, and in the transfer process, the toner image on the image bearing member is transferred to a transfer material such as a sheet of paper and the like. Further, in the fixing process, the toner image on the transfer material is fixed, and in the cleaning process, a residual matter such as a transfer residual toner remaining on the surface of the image bearing member after the toner image is transferred is removed.
In the above cleaning process, the transfer residual toner collected by a cleaning device is contained in a disposable container, and thereafter, has been discarded. Moreover, in recent years, a cleaner-less process has been proposed in which a cleaning device is omitted in order to miniaturize an image forming apparatus, and maintenance required for discarding a transfer residual toner is dispensable. According to one embodiment of the cleaner-less process, a developing device attracts a toner to a portion of the image bearing member in which a surface potential decays by exposure by reversal development while collecting a residual matter remaining on a non-exposure portion. More specifically, after the transfer process, the residual matter passes through the charging process, and thereafter, is collected to the developing device by an electrostatic force generated by the difference (hereinafter, referred to as xe2x80x9cback contrastxe2x80x9d) between a surface potential of the image bearing member and a developing bias applied to a development roller.
According to the above-mentioned type charging device, in the transfer process, a residual matter on the positively charged image bearing member and a reversal toner remaining in a non-image section are attracted to a charging roller used as a charging rotary member; for this reason, a charging performance of the charging roller is reduced. Moreover, in the image forming apparatus using the cleaning device, a toner passing through the cleaning device is attracted to the charging roller; as a result, a charging performance of the charging roller is reduced, although it is not so conspicuous as the cleaner-less type.
Conventionally, in order to prevent the reduction of a charging performance, various types have been proposed, such as:
a type of providing a collecting member for temporarily collecting a reversal toner or the like upstream of the charging roller;
a type of abutting a cleaner member against the charging roller; and
a type of collecting a toner onto the image bearing member by mechanical rubbing in a state that a peripheral speed is given to the charging roller.
However, in the above types, that is, the type of providing a collecting member for temporarily collecting a reversal toner or the like upstream of the charging roller and the type of abutting a cleaner member against the charging roller, there is the following problem. More specifically, another member is required; for this reason, the cost increases. In addition, the effect cannot be sufficiently obtained when the reversal toner is accumulated in the collecting member and the cleaner member.
Moreover, in the above type of giving a peripheral speed different from the image bearing member to the charging roller, there is the following problem. That is, although the toner may be returned by the above mechanical rubbing, under an electric field condition applied to one direction such as DC charging, the toner, which is not collected to the image bearing member by only mechanical rubbing, is attracted to the charging roller depending on environment and the using condition of printer. When the toner is attracted to the charging roller, a resistance of the charging roller becomes locally high; for this reason, an excessive discharge occurs when contamination is little, and further, when the contamination exceeds a predetermined value, a charging failure occurs. As a result, a charging potential on the image bearing member falls into disorder as shown in the following Table 1, and thus, image uniformity can not be obtained by the above disorder of the charging potential. Moreover, under the condition of high temperature and high humidity, the toner is not likely to retain a charge; for this reason, this is a factor of increasing an extraneous amount onto the cleaning roller of the reversal toner and the residual matter on the image bearing member. As a result, the charging performance is reduced in the print endurance latter by half, and thus, image failure occurs.
The present invention has been made in view of the above problems. Accordingly, an object of the present invention is to provide a charging device and an image forming apparatus, which can prevent toner contamination of a charging rotary member.
Another object of the present invention is to provide a charging device and an image forming apparatus, which can obtain a charging potential stable as a surface potential of an image bearing member.
Another object of the present invention is to provide a cleaner-less type image forming apparatus having no dedicated cleaner, and a charging device used for the image forming apparatus.
Another object of the present invention is to provide a charging device and an image forming apparatus, which can frictionally charge a toner to a normal polarity by a charging rotary member.